The Richard C. Wood U.S. Pat. No. 4,720,396, issued Jan. 19, 1988 describes a new system for solder finishing integrated circuit package leads. The machine and method described in U.S. Pat. No. 4,720,396 is directed to solder finishing the leads of a dual-in-line (DIP) integrated circuit package having two downwardly directed parallel rows of leads along the sides of the package. In a DIP the leads of the two parallel rows of leads are folded or bent in a downward direction at right angles to the body of the DIP package.
The solder finishing apparatus in U.S. Pat. No. 4,720,396 establishes two vertical falling columns of falling molten solder spaced apart a distance substantially the width of the package. The DIP package passes between the vertical falling columns along a horizontal track section immersing the two parallel rows of leads along the sides of the package in the respective falling columns of molten solder. The falling columns of molten solder wash the downwardly directed leads of the two rows of leads, washing the leads and depositing a finish layer. Hot gas is directed over the leads of the package as they emerge from the falling columns to eliminate excess solder and bridging of solder between the leads. The monorail track system and the solder bridge track section which defines the locus of solder finishing are horizontally oriented to accommodate the DIP package configuration.
The method and apparatus of U.S. Pat. No. 4,720,396 contemplate solder finishing conventional DIP packages passing along a horizontal track with depending leads directed downwardly. The falling columns of molten solder simultaneously encounter the two rows of leads at this generally vertical downwardly directed angle so that solder runs down the leads under gravity to the solder pot below. This coincident angle of encounter enlists gravity to minimize excess accumulation of solder, solder bridging, and splashing or spattering.
While U.S. Pat. No. 4,720,396 provides a successful solder finishing system for DIP package leads, it did not anticipate the problem of solder finishing flat pack integrated circuit parts with rows of coplanar or axially directed leads. In a flat pack or flat package IC device the leads extend outwardly in the same plane as the body of the flat package. The leads thus lie in the horizontal plane during travel along the track of U.S. Pat. No. 4,720,396. As a result, the columns of falling molten solder encounter horizontally directed leads at right angles rather than at the preferred downwardly depending angles contemplated by U.S. Pat. No. 4,720,396. Gravity does not enhance runoff to minimize solder accumulation and solder bridging, and there is increased splashing and splatter.